Azure provider: support most recent gwacl
This branch updates the Azure provider code to cope with the latest version of gwacl. Two changes in gwacl require the provider code to be updated:
- 'UserData' has been renamed 'CustomData' in the linux configuration object.
- gwacl no longer uses go-curl but instead uses a forked crypto/tls package which supports TLS renegociation. This requires a couple of changes in the tests because the crypto/tls package requires a valid certificate.
https://code.launchpad.net/~rvb/juju-core/latest-gwacl/+merge/176211
(do not edit description out of merge proposal)